The present invention relates to the mapping of an underground cable or pipe. It also relate to an interrupter for interrupting the cathodic protection voltage applied to such an underground cable or pipe.
It is well known to apply a cathodic (negative) voltage to an underground cable or pipe (hereinafter xe2x80x9cpipexe2x80x9d) to reduce corrosion of that pipe. If the pipe is at a positive voltage relative to ground, electrolytic effects occur which damage the pipe. It should be noted that such cathodic protection voltage may be applied even when the pipe is coated to insulate it from the ground, because it is common for that insulation to develop holes or other faults which could result in localised damage.
However, it is also common for such pipes to experience electrical fields due to other objects such as adjacent pipes, or other electrical conductors such as railway lines, etc. Such localised fields sometimes result in the pipe experiencing a positive voltage relative to ground, so that corrosion occurs. It is therefore desirable to investigate currents in the pipe or cable to detect points where corrosion may occur. If stray currents are detected, these may be due to electric fields from other objects (such currents hereinafter being referred to as stray currents), appropriate action can be taken, such as repairing the fault or taking corrective action elsewhere by suitable voltage control, or even by providing sacrificial anodes at an adjacent fault.
In order to detect the stray currents, it is possible to make use of the magnetic fields generated by those currents, and detect those fields remotely from the pipe itself, such as at the surface. Detection of such magnetic fields is generally known, although special techniques may be needed because of the low magnitude of the currents.
There is also the problem that there may be several pipes in the area being investigated, and the currents from those other pipes may confuse the measurement.
Therefore, it is preferable that the cathodic protection voltage is modulated to enable the currents generated thereby to be more easily recognised by remote detectors. At its simplest, the modulation could be applied by a relay controlled switch connected between the pipe and the voltage source which supplies the cathodic protection voltage. However, the present invention, in its several aspects, seeks to develop stray current mapping arrangements, and also to provide an interrupter for applying a modulation to the cathodic protection voltage, and hence to the stray currents, and to improve the investigation of those stray currents.
Before discussing these aspects of the invention, however, it needs to be borne in mind that one source of stray currents is due to the cathodic protection voltage applied to other pipes which pass adjacent or across the pipe being investigated. Stray currents on the pipe being investigated may thus have multiple components, particularly when there are a large number of pipes adjacent each other.
Thus, a first aspect of the present invention proposes that the cathodic protection voltage of the pipe being investigated is modulated with a first modulation signal, and the cathodic protection voltage of a second pipe which passes adjacent the first pipe is modulated with a second modulation signal. Then, the stray currents on the pipe being investigated are analysed on the basis of the different modulation signals applied to the two pipes. If the stray currents are analysed at the first modulation, those current components due to the pipe itself, or due to other perturbations such as electric railways are determined. Then, if the stray currents are investigated at the second modulation, ie the modulation applied to another pipe, the effect of the linking between that other pipe and the pipe being investigated can be determined. This process can be repeated for multiple pipes by modulating with a distinct signal for each pipe.
Where the cathodic protection voltage of two pipes is to be modulated, whether it be the same modulation signal or with different modulation signals as in the first aspect, separate interrupters will be fitted to each pipe, and although the interrupters may be driven by oscillators having the same nominal frequency, manufacturing tolerances etc mean that synchronisation cannot be reliably achieved. Therefore, the interrupters need an additional system to provide synchronisation. The synchronisation represents a second aspect of the present invention.
Synchronisation could be achieved by directly connecting the two interrupters, so that the clock signal of one can be used as a reference against which the other is synchronised. However, in many situations, it is not practical for the interrupters connected to different pipes themselves to be connected. The pipes may only pass adjacent each other at a particular point along their length, and if the interrupters cannot be connected at that point, physical interconnection is problematic. Therefore, in a third aspect of the invention, synchronisation makes use of an arbitrary pre-set time for all interrupters. When any individual interrupter starts to operate, after it has stopped operating for some time, it determines the time interval between that pre-set time and the time of start of operation, and the interrupter signal (which is modulated by an appropriate modulation signal), is commenced at the point in the signal which corresponds to that which the signal would be if the signal had commenced at the pre-set time. In this way, the interrupters are synchronised as if any operating interrupter had started at the pre-set time, irrespective of the time which has past since that pre-set time.
In order for this operation to occur, the interrupter needs to determine the time between the pre-set time and the time at which the interrupter starts operation. If an absolute reference clock was available for each interrupter, that reference clock could be used. However, it is not normally economic to provide such accurate time measurement within an interrupter. Therefore, it is preferable that the interrupters make use of an external signal. If an external time signal is available (such as the Rugby signal in the UK), then that could be used. A further alternative is to make use of the Global Positioning System (GPS). Whilst those signals are primarily to provide positional information, they also provide a synchronised clock signal from which the interrupters can determine the time between the pre-set time and the time of starting the interrupter, and so can determine at which point in the interrupter signal the operation is to start.
To put this aspect another way; if all interrupters operated continually, synchronisation would be achieved by starting them all at the same time (the pre-set time). However, since the interrupters are to be turned on and off, a calculation is made whenever they are turned on to determine the point in the cycle they would have reached if they always had been turned on (from the pre-set time referred to above), and the signal is started at the appropriate time in the cycle corresponding to that which would have occurred if the interrupter had been on all the time.
Another aspect, which applies to the modulation of any single pipe, as well as modulations applied to multiple pipe as discussed above, is that the modulation is preferably an irregular modulation, rather than simple regular square-wave modulation. This makes the stray currents being investigated easier to distinguish from other currents in the vicinity of the pipe being investigated.
Since the currents on one pipe may be due not only to the modulation applied to that pipe, but also to the modulation applied to other pipes, it is important that the resulting field generated by the combinations of those modulations must be such that the individual modulations should be separately identifiable. It is possible to do this by modulating at different frequencies, but this has high power requirements. Therefore, in a further aspect of the present invention, it is proposed that the different pipes are modulated with signals which are a sequence of binary levels defining a bit pattern, with the bit pattern of each signal being orthogonal to all other patterns. Use of this aspect means that the presence of one pattern, and hence one signal, can accurately be determined and measured even in the presence of any or all of the other patterns and hence all other signals.
Orthogonality can be expressed mathematically as: S1(t).S2(t)=0 where S1 and S2 are the signal waveforms, represented as functions of time. The integral is taken over a period of time equal to the repeat period of the waveforms. If the equation holds true then the two signals S1(t) and S2(t) are orthogonal, provided neither signal is trivial, i.e. S12(t) 0 and S22(t) 0
Thus, this use of orthogonal signals represents a fourth aspect of the invention.
Even when the pipes are carrying orthogonal signals, erroneous results may be obtained if measurements are carried out in a location where two pipes are closely adjacent, since the signal in one pipe may couple to the other. When measurements are carried out remotely which seek to measure the modulation on one of those pipes, the same modulation may appear on the other pipe due to coupling, and therefore the magnetic field being measured will represent the sum of the fields from those two fields. Therefore, it is necessary to separate them.
To do this, it is proposed that one of the pipes be isolated and signals applied to it to determine its position. Then, after returning it to the non-isolated state, the net signals from the two pipes are investigated. Since the location of one of the pipes is known, it is then possible to determine the location of the other by subtracting the fields that would be generated by the pipe at the known location, and then analysing the resultant fields which then represent the second pipe. This subtraction process represents a fifth aspect of the invention.
Normally, in order to make for the use of the present invention, the interrupter includes a switch in the form of a solid-state device rather than a relay-controlled switch. It has been appreciated that it is then possible to make use of the cathodic protection voltage to provide power, instead of, or as a supplement to battery power. This represents a sixth aspect of the invention. In the sixth aspect, the interrupter has power storage (eg a capacitor) connected between the cathodic protection supply and ground. In the normal state, where the interrupter is not interrupting the cathodic protection voltage, a voltage is present across the power storage means which thus stores power which can be used to operate the interrupter when the interrupter is to function.
Indeed, this sixth aspect of the invention is not limited to the powering of an interrupter. Any suitable device connected to the pipe may thus be powered by the cathodic protection voltage.
As has been mentioned previously, the stray currents usually have small magnitudes, and therefore it is desirable to be able to detect those small currents remotely. To detect those currents, the detector apparatus disclosed in WO98/54601 may be used in which a plurality of sensors are provided, normally in a horizontally-extending array. Then, comparison of the sensor outputs enables the position of the pipe to be determined accurately. The sensors detect the magnetic fields generated by the currents, and the arrangements described in WO98/54601 permit detection even at low frequencies.
The final aspect of the present invention concerns the mapping of stray currents themselves. When an attempt is made to determine the position of an underground pipe using a detector with multiple sensors, the effect of an interfering adjacent pipe (a second pipe) is to distort the signals received by the sensors, so that it is not possible to determine from the signals from the sensors the location of the pipe being investigated (the first pipe).
Suppose now that a counter-interference is supplied to signals from the sensors, by modifying them as if there was an interfering pipe at a set location, and the current in that interfering pipe was varied. If this process is carried out until the outputs of the sensors are modified so as to coincide (or nearly so) at a particular point. Then, the xe2x80x9cvirtualxe2x80x9d interference would have cancelled out the actual interference from the second pipe. At first sight, such cancellation would require the position of the virtual pipe to be set at the same place as the second pipe, but this turns out not to be the case. The effect of the second pipe at one location can be cancelled by a virtual pipe at another location, with a difference current. Thus, it is not necessary that the position of the virtual pipe is set accurately. Indeed, it could be present at some fixed location and sufficiently accurate results would normally be obtained. Of course, if the operator knew approximate the location of the second pipe, then greater accuracy could be achieved, but again the position needs only to be known approximately, not exactly, for sufficiently accurate results to be achieved. This use of a virtual interfering pipe to correct the distortions in sensor signals due to a real interfering pipe thus represents a seventh aspect of the invention.